Just as a slight warning, I just checked the CPU support list for the asrock b450 Pro 4, and it's got the 3800x but, notably, not the 3900x. Might be an oversight, but might also be that not all the CPUs actually work on all the boards even if they have VRMs that should be able to cope.
Also from the few 400 series boards I've checked, none list the cheap A series APUs as supported, so anyone thinking of grabbing one of those for a cheeky bios update might want to hold fire & let someone else be the guinea pig.